Horror movie Mama may sound on paper like every other horror movie you've seen in the past couple of years, from Woman in Black to Insidious. Thankfully, it isn't. Not only is Mama pretty damn spine-tingling, but it also has characters whose roles involve more than screaming and looking slack-jawed or bug-eyed or any number of other "OMG I saw a scary" expressions. Plus, it obeys the one rule of monster movies that guarantees an interesting ride.

Very light spoilers ahead.

I was dreading having to watch Mama, despite the fact that Guillermo Del Toro produced it, because the plot sounded so generic. Ready? There is a lady ghost (the eponymous mama) who is sad and angry about her dead baby. Yes, you have seen/heard/read that ghost story possibly thousands of times. But Mama offers a fantastic twist. First of all, right from the very beginning, the ghost isn't really the bad guy. The movie opens with a horrific murder in the wake of a financial scandal we hear about in snatches of radio chatter. Jeffrey (Game of Thrones's Nikolaj Coster-Waldau) owns a company that has lost everything in the scandal. He's gone nuts and killed his business partners, then murdered his estranged wife and kidnapped their two young daughters.

Jeffrey takes the little girls, Lilly and Victoria, out to a remote cabin where he plans to finish his murder spree sundae with a suicide on top. We get little hints that the cabin is occupied — there are little ghosty zooms at the corner of the frame that are familiar from every haunted house movie. But as I said, the ghost isn't the real monster here. It's Jeffrey. As he raises the gun to Victoria's head, the ghost appears in a smoky haze, eats that murdering bastard alive, and rescues the two little girls.

That's just the first few minutes. But they're all you need to know you're watching a very promising horror movie. The plot isn't going to devolve into a rote game of "is there a ghost or isn't there?" Yes there is a freakin ghost, and the interesting questions involve who she was in life, who she has become, and what kind of relationship she'll form with these two extremely young and helpless girls.

Ultimately, the movie is about the ghost's relationship to the girls, who aren't discovered for five years. Jeffrey's twin brother Lucas, an artist, has used his brother's remaining money to fund a search for Lilly and Victoria in the vast wooded area where Jeffrey was last seen. When they're finally discovered, looking exactly like feral ghost children, Lucas is living with his bass-playing girlfriend Annabel (Jessica Chastain, awesomely channeling Joan Jett). Annabel has just taken a pregnancy test and is jumping around celebrating the fact that she isn't pregnant. So the two of them aren't exactly prepared to care for two deeply-disturbed little girls who have been living "alone" in the forest for half a decade.

Still, with the help of a local psychiatrist, they decide (unwillingly, in Annabel's case) to become surrogate parents to Lilly and Victoria. Which means that they have also brought the ghost into their lives as well. There are dozens of creepy scenes as Annabel slowly realizes that the girls have been in the care of a ghost. As she learns more about what's happened to both the girls — and the woman who became the ghost — she slowly comes to love the children. But she also comes to sympathize deeply with the ghost, despite the ghost's habit of maiming and killing anyone who gets between her and the girls.

The parallel development of Annabel and the ghost's characters makes for a fascinating movie, with an ending that is satisfyingly unconventional — though it isn't entirely original either. In fact, no single element in this movie could be described as original. There is something delightfully old-fashioned about the ghost, whose crooked body and floating hair look like something out of a 1930s pulp. And as I said, the bare-bones outline of the plot resembles a lot of other movies. Still, it is so artfully put together, with well-observed set pieces and good acting, that you'll be invested in the story for reasons that go way beyond its ability to scare you.

After all, making you jump or cover your eyes in a movie is as easy to do as making somebody pop wood when they watch a bad porno. No points for that. But making you care about the characters, and ponder why they are behaving a certain way? That's good storytelling. One of the main reasons Mama is successful is that it presents us right away with a world where the supernatural is real, ghosts are real, and therefore we neatly sidestep the boring questions that preoccupy the entirety of a movie like Woman in Black.

The pleasure of supernatural horror is in discovering another world where the rules of reality don't apply. A world where a wronged woman can finally overcome her traumas, even after death. A world where fathers aren't allowed to murder their little girls. And a world where even wildly non-traditional mothers can be sources of love and protection. Mama takes us into that world, for just a little while. It will make you shiver with fear, but it might also make you question what passes for "natural" when it comes to motherhood.